Dielectric insulation media in liquid and/or gaseous states (i.e. fluids) are widely applied to insulate an electrically active part in a variety of electrical apparatuses, such as switchgears or transformers. For example, the electrically active part in medium or high voltage metal-encapsulated switchgear is arranged in a gas-tight compartment which encloses an insulation gas with a pressure of several bars which electrically separates the compartment of the apparatus from the electrically active part. In other words, the insulation gas does not allow the passage of electrical current from the electrically active part to the compartment. A commonly used dielectric insulation gas is sulfur hexafluoride (SF6) which exhibits excellent insulation and electric arc extinguishing capabilities. However, SF6 is a strong contributor to the green-house effect and thus has a high global warming potential. Therefore alternative insulation fluids should be found.
Several alternative insulation fluids have been identified. Some of these alternatives comprise multi-component fluid mixtures, i.e. they comprise more than one molecular or atomic species. It is found that certain properties of such insulation fluid mixtures are viable for the safe operation of the electrical apparatus. As an example, the dielectric breakdown strength of the insulation fluid is strongly dependent on the concentration ratio of the mixture components and on the total fluid pressure. In order to maintain the mixture's insulating features and thus the safety and functionality of the electrical apparatus, the concentrations of the different components of the insulation fluid and the total number of particles in the fluid must remain constant or at least within certain boundaries. For this, sensor devices are used to monitor the physical state of the insulation fluid.
For electrical apparatuses insulated with a single component gas such as SF6, such a monitoring can be achieved by measuring the fluid density inside all gas-filled compartments of the electrical apparatus. Because the density of a gas inside a closed volume is independent of pressure and temperature, a density measurement gives a direct indication of whether, e.g., a leakage fault is present or not. For insulation fluid mixtures, a density measurement alone does not suffice, because the dielectric insulation strength additionally depends, e.g., on the ratio of the components.
US 2002/0095262 A1 and U.S. Pat. No. 7,184,895 B2 describe methods and devices for monitoring the proportion of a component in a gaseous insulation medium consisting of at least two components.
The disclosed methods and devices have the disadvantage, however, that they do not determine and/or distinguish fault scenarios for the insulation media that can compromise the safe operation of the electrical apparatus.
Hillers et al., “Control, monitoring and diagnostics for high voltage GIS”, IEE Colloqium on GIS at transmission and distribution voltages (Digest No. 1995/293), 14 Nov. 1995, Nottingham, IEE London (UK) discloses GIS monitoring based on SF6 Gas monitoring, optionally in combination with partial discharge measurement and arc detection.
The Alstom product brochure “CBWatch-2 Modular circuit breaker monitoring system” discloses circuit breaker trend monitoring based on measurement of total gas pressure and gas temperature and subsequent calculation of gas density. As well, liquefaction, SF6 leakage rates, close and open operating times, contacts separation speed, and other circuit breaker and drive variables can be watched.